Nanometer-scale sharpening of conductor tips

ABSTRACT

The invention provides methods for sharpening the tip of an electrical conductor. The methods of the invention are capable of producing tips with an apex radius of curvature less than 2 nm. The methods of the invention are based on simultaneous direction of ionized atoms towards the apex of a previously sharpened conducting tip and application of an electric potential difference to the tip. The sign of the charge on the ions is the same as the sign of the electric potential. The methods of the invention can be used to sharpen metal wires, metal wires tipped with conductive coatings, multi-walled carbon nanotubes, semiconducting nanowires, and semiconductors in other forms.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application60/794,924, filed Apr. 26, 2006, which is hereby incorporated byreference to the extent not inconsistent with the disclosure herein.

ACKNOWLEDGEMENT OF GOVERNMENT SUPPORT

This invention was made at least in part with support from the Office ofNaval Research under grant numbers N00014-03-1-0266 and N00014-06-10120.The United States Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

Electrical conductors with ultrasharp tips have applications as probesfor scanned probe microscopy and field emitters for use in scanningelectron microscopy (SEM), transmission electron microscopy (TEM) andfield emission displays. In probe microscopy, the sharpness of the tipaffects the lateral resolution. For field emission, the sharpness of thetip affects the electric field at the tip.

A variety of techniques have been reported for producing sharp tips onelectrical conductors, including electrochemical etching, chemical vapordeposition or electron beam deposition onto previously sharpened tips,and ion sputtering. Electrochemical etching is a common technique usedto produce sharpened tips on wires of tungsten and other materials.Typically, the radius of curvature of the apex of the tip is about 1micron or less. Electrochemically sharpened tungsten tips typically havean oxide layer present on the tip surface.

Several ion sputtering techniques have been described in the scientificand patent literature. Biegelson et al. (1987, Appl. Phys. Lett, 50(11)696) report a technique in which a beam of energetic ions is directedtowards an electrochemically etched tungsten tip at an angle withrespect to the tip axis. The tip is then rotated within the ion beam,resulting in sputter removal of the oxide layer and reduction of theradius of curvature at the tip apex. U.S. Pat. No. 6,329,214 to Hattoriet al, report ion milling of noble metal field emitters with an ion beamincidence angle of 30-60 degrees relative to the substrate normaldirection.

Axial incidence ion beam sputtering techniques have also been reported.U.S. Pat. No. 5,993,281 to Musket describe sputtering by high-energy (30keV and higher) ions incident along or near the longitudinal axis of afield emitter to sharpen the tip with a taper from the point, or topend, down the shank of the emitter. The process is reported to sharpentips down to radii of less than 12 nm with an included angle of about 20degrees. U.S. Pat. No. 6,329,214 to Hattori et al. reportion milling ofemitters made of conductive material other than a noble metal with anion beam incidence angle of zero degrees relative to the substratenormal direction. Kubby and Siegel (1986, J. Vac. Sci. Technol. B 4(1),120) reportion milling of electropolished tungsten and iridium wiretargets; the target was electrically isolated from the target chamberand mechanically rotated and the beam energy was in the interval 3-15keV. Morishita and Okuyama (1991, J. Vac. Sci. Technol. A 9(1), 167)report sharpening of monocrystalline molybdenum tips with Ar⁺ or Xe⁺ions focused into a beam approximately 350 microns across. Hoffrege etal. (2001, J. Appl. Phys., 90(10) 5322) describe variation of the anglebetween the ion beam and the macroscopic tip.

Focused ion beam (FIB) milling techniques have also been reported.Typical beam diameters (full width at half maximum) are from about 5 nmto about 1 μm. Vasile et al. describe FIB milling of electrochemicallyetched W and Pt—Ir tips via a three stage process (Vasile et al., 1991,Rev. Sci. Instrum., 62(9), 2167; Vasile et al., 1991, J. Vac. Sci.Technol. B 9(6)). Formation of microtips having radii of curvaturebetween 4 nm and 30 nm was reported (Vasile et al., 1991, Rev. Sci.Instrum., 62(9), 2167). U.S. Pat. No. 5,727,978 to Alvis et al.describes FIB milling of platinum deposited on an electron beam emittingfilament.

Self-sputtering sharpening techniques are also described in thescientific literature. As reported by Schiller et al. (1995, SurfaceSci. 339 L925-930), electrochemically etched tips are placed in withinan ambient neon environment and a high negative voltage is applied tothe tip. Under such a voltage, electrons are emitted directly from thetip, impacting and ionizing surrounding neon ions. These positivelycharged neon ions are attracted to and sputter the tip. The sputteringprocess results in “necking” and then “decapitation” of the tip.

There remains a need in the art for methods for producingnanometer-scale conducting tips, especially methods which areself-limiting and capable of sharpening more than one tip at a time.

SUMMARY OF THE INVENTION

One aspect of the invention provides methods for sharpening the tip ofan electrical conductor. Since the sharpened tips can be used as probesor field emitters, some embodiments of the invention also providemethods for sharpening the tips of probes or field emitters. The methodsof the invention are capable of nanometer-scale sharpening, producingultrasharp tips having an apex radius of curvature less than 10 nm.Typically, the tips produced by the methods of the invention have anapex radius of curvature less than 5 nm. The sharpening methods of theinvention can be self limiting, so that the tip shape eventuallyapproaches an equilibrium value. The self limiting nature of the processcan obviate the need for careful monitoring of the process and the needfor expensive monitoring apparatus.

The sharpening methods of the invention are based on simultaneousdirection of ionized atoms towards the apex of a previously sharpenedconducting tip and application of an electric potential difference tothe tip relative to some reference potential (such as the potential ofthe surrounding vacuum chamber, which may be electrically grounded). Thesign of the charge on the ions is the same as the sign of the electricpotential difference. Application of an electric potential to the tipgenerates an electric field around the tip, with the strength of thefield varying inversely with the tip radius of curvature. Because theelectric field surrounding the tip is non-uniform and dependent on thetip form, the flow of ions is modified by the shape of the tip apex.This results in a selective repulsion of ions from the tip apex and amodification of the angle at which ions impact the tip. As sharpeningproceeds, the apex radius of curvature is further reduced, which furtherincreases the local electric field strength and enhances the sharpeningeffect. This process may be termed “field-directed sputter sharpening.”

For a given conductor material, type of ion and ion angle of incidence,the accelerating voltage of the ions and the voltage applied to theconducting tip are selected together to provide the desired tipsharpness. The difference between the ion accelerating voltage and thetip voltage is sufficiently large that sputtering of the tip occurs, butnot so large that the influence of the tip voltage is negligible.Simulations of the sputtering process can be used to aid in selection ofthe accelerating voltage and the voltage applied to the sharpenedconductor.

In an embodiment, the invention provides a method for sharpening the tipof an electrical conductor comprising the steps of:

-   -   a. providing a vacuum in a vacuum chamber;    -   b. providing a conductor comprising a tip having an initial        radius of curvature less than 1 micron at its apex, the        conductor being located within the vacuum chamber; and    -   c. simultaneously applying a voltage to the conductor and        directing a flux of ions onto the tip of the conductor, the ions        being characterized by an acceleration voltage, wherein the sign        of the voltage applied to the conductor is the same as the sign        of the charge of the ions,        wherein the acceleration voltage and the voltage applied to the        conductor are selected so that the tip of the conductor is        sputtered by the ions, thereby reducing the radius of curvature        at the apex of the tip.

In one aspect of the invention, the final radius of curvature at theapex of the tip is less than about 5 nm. In an embodiment, the ionacceleration voltage is between about 550 eV and 5 keV and the voltageapplied to the conductor is 100 V or more. In an embodiment, at leastsome of the ions have an angle of incidence with respect to thelongitudinal axis of the conductor of less than or equal to 35 degrees.

In the methods of the invention, an ion source (for example, a plasma)may be used to generate ions. A flux (or flow) of ions may be obtainedwhen the ions from the ion source are accelerated by means of anelectric potential difference, referred to herein as the accelerationvoltage. The electric field associated with this electric potentialdifference affects the trajectory of the ions. A variety of ions areknown to those skilled in the art.

In an embodiment, the ion flux is provided by a collimated beam of ions.In one embodiment, the longitudinal axis of the ion beam issubstantially aligned with (within 5 degrees of) the longitudinal axisof the conductor. In this embodiment, the conductor need not be rotated.In another embodiment, the longitudinal axis of the ion beam may be at anon-zero angle to the longitudinal axis of the conductor. In anembodiment, the angle between the ion beam and longitudinal axis of theconductor is less than or equal to 35 degrees. In this embodiment, theconductor may be rotated to sharpen the conductor more uniformly. Inanother embodiment, a focusing ring can be used to cause ions to impingeupon the conductor at an angle off the longitudinal axis.

In another embodiment, the ion flux is not in the form of an ion beamand the ions have a greater variation in the angle of incidence withrespect to the conductor. In an embodiment, the electric field linesassociated with the accelerating voltage are substantially aligned withthe longitudinal axis of the conductor. In another embodiment, the anglebetween the electric field lines and longitudinal axis of the conductoris less than or equal to 35 degrees. In another embodiment, at leastsome of the ions have an angle of incidence with respect to thelongitudinal axis of the conductor of less than or equal to 35 degrees.Batch processing in chambers that produce large area ion fluxes may tunethe distribution of ion incidence from on axis to random.

In an embodiment, the spot size of the ion beam or the area of the ionflux is sufficiently large to sputter a plurality of conductorssimultaneously. The ability to sputter multiple tips in parallel wouldpotentially allow for dozens or hundreds of tips to be simultaneouslyprepared, significantly increasing production throughput.

The invention also provides ultrasharp probe and field emitter tips. Inan embodiment, probe or field emitter tips have an apex radius ofcurvature less than 2 nm. In an embodiment, the tip is not a singlecrystal metal tip. In another embodiment, the probe or field emitter tipis made of a material other than tungsten. The ultrasharp probesprovided by the invention may be used in scanning probe microscopy.

The invention also provides apparatus which enable sharpening ofconductor tips using the methods of the invention. The apparatus may bestand-alone. The apparatus may also be a scanning probe microscopeapparatus which allows in situ sharpening of the probe tip or anelectron microscope apparatus which allows in situ sharpening of thefield emitter.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1: Schematic illustrating structural features of a sharpened tip.

FIG. 2A: Ion path simulation results for a neon ion beam aligned withthe longitudinal axis of the conductor. The neon atoms are singlyionized with an energy of 2000 eV, while the bias applied to the tip is100V.

FIG. 2B: Ion path simulation results for a neon ion beam aligned withthe longitudinal axis of the conductor. The neon atoms are singlyionized with an energy of 2000 eV, while the bias applied to the tip is400V.

FIG. 2C: Ion path simulation results for a neon ion beam aligned withthe longitudinal axis of the conductor. The neon atoms are singlyionized with an energy of 2000 eV, while the bias applied to the tip is100V.

FIG. 3: Simulated field-directed sputtering under 400V tip voltage, 2000eV argon ion energy, and beam aligned with the longitudinal axis of theconductor.

FIG. 4: Simulated field-directed sputtering for zero tip voltage, 1600eV argon ion energy, and beam aligned with the longitudinal axis of theconductor.

FIGS. 5A-5F: TEM images of a Pt—Ir tip prior to sputtering (FIG. 5A) andat various stages of the sputtering process (FIGS. 5B-5F).

FIGS. 6A-6B: TEM image of a W tip prior to sputtering (FIG. 6A) andfollowing sputtering (FIG. 6B).

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic illustration of a conductor 14 having alongitudinal axis 14′. The conductor also has a tip; the term “tip” asused herein refers to a pointed or narrowed end. The tip 21 of theconductor tapers from its apex 20 to its base 23. The cross-sectionaldiameter or width 24 of the tip base 23 is the same as that of conductorshank 22 (which is unchanged from its initial value). The tip length 26is the distance between the apex 20 and base 23. The tip apex has aradius of curvature 25 (distance between the arrows in FIG. 1). The tipmay also be characterized by the included angle or cone angle defined bythe tapered surface of the tip (indicated by θ in FIG. 1). When the tipgeometry is more complicated than that shown in FIG. 1, the cone anglemay vary along the tip length and/or multiple apices may be present. Thecone angle may be determined near the tip apex or at a specifieddistance from the tip apex. At a specified distance from the tip apex,the cone angle may be estimated as twice the angle whose tangent is halfthe tip width at the specified distance divided by the specifieddistance. If multiple apices are present, the primary apex is that whichis most centrally located. The measurement of a tip's radius ofcurvature can be achieved by fitting a circle within the apex within aTEM image and measuring the radius directly.

As used herein, sharpening a tip includes reducing the radius ofcurvature at the tip apex. In an embodiment of the invention, theinitial radius of curvature of the tip apex is less than about 1 micron(this is the tip radius before field-directed sputter sharpening). Tipswith this initial radius of curvature may be obtained by electrochemicaletching or any other suitable method known to those skilled in the art.In different embodiments, the methods of the invention allow reductionof the radius of curvature at the primary tip apex to less than 5 nm,less than or equal to 2 nm, less than or equal to 1.5 nm, or less thanor equal to 1 nm.

In an embodiment, sharpening of the tip also leads to reduction of thecone angle as determined near the tip apex. Without wishing to be boundby any particular belief, it is believed that smaller cone angles can beachieved if the ion flux or beam is not strictly aligned with the tip.The sharpening process of the present invention may also lead to adecrease in the overall length of the conductor, an increase in the tiplength, and/or a reduction in the cone angle as determined at otherlocations along the tip length.

In the sharpening methods of the invention, the electric field aroundthe conductor tip is used to direct ions in the vicinity of the tip andthereby control sputtering of material from the tip. In particular, thepath of these ions is affected by the repulsive force generated betweencharge stored within the conductor and ions in the vicinity of theconductor; the magnitude of the force depends on the magnitude of theelectric field. Typically, the electric field around the conductor willchange during the sharpening process, with enhancement of the field nearthe tip apex as the tip sharpens.

For a given tip bias voltage, the shape of the sharpened tip approachesan equilibrium shape after extended sputtering. Therefore, the processcan be self-limiting. The equilibrium form of the sputtered tip dependson several parameters. These include the relation between angle ofincidence and sputter yield (a property of the selected tip material andion), the angle of the incoming ions, and the bias applied to the tip asit relates to the ion energy.

The electric field around the conductor tip depends on the electricpotential applied to the conductor relative to a reference potential(such as the potential of the vacuum chamber). This electric potentialdifference can also be termed the applied voltage or the applied biasvoltage. One upper limit on the absolute value of the applied potentialis the acceleration voltage minus the threshold value for sputtering tooccur. In an embodiment, the upper limit on the absolute value of theapplied potential is the accelerating voltage less approximately 500 V.In different embodiments, the absolute value of the applied voltage isgreater than 25 V, greater than 50 V, greater than 100 V, greater than200 V, greater than 300 V, greater than 400V, greater than 500V, greaterthan 750 V, or greater than 1000 V. In different embodiments where theion accelerating voltage is between 1 keV and 5 keV or approximately2000 eV, the absolute value of the applied potential is between 100 and700V, between 200 and 600 V, or between 300 and 500V.

The applied voltage may be substantially constant or varying. Indifferent embodiments, the variation in voltage is less than 5% or lessthan 1%. The voltage variation may take the form of a series of constantvoltages applied to the conductor. For example, the first voltageapplied may be higher (e.g. 600-800 V) and the second voltage appliedmay be lower (e.g. 100-400V). A multi-stage process may also use atleast one stage with a constant voltage and at least one stage with anon-constant voltage. For example, application of a constant voltage maybe followed by application of a varying voltage to the tip. Thepotential may be applied by any means known to those skilled in the art.For example, the potential may be applied using a voltage supply.Alternately, the potential difference may be generated automatically byusing a resistor to electrically separate the conductor from the vacuumchamber. The ion flux would then charge the conductor up to somepotential.

The difference between the accelerating voltage of the ions and thepotential applied to the conducting tip is sufficiently high thatsputtering occurs when the tip is contacted with the ion flux. A lowerlimit on the difference between the acceleration voltage and the tipbias is that this difference is greater than or equal to the thresholdenergy for sputtering. In an embodiment, this difference is great enoughthat the sputtering yield is greater than about 0.001. Relatively lowsputtering yields can be compensated by relatively high ion currentdensities. In other embodiments, the sputtering yield is greater than0.25, greater than 0.5 or is approximately 1. In different embodiments,the difference between the ion accelerating voltage and the potentialapplied to the conductor is greater than or equal to 100 eV, greaterthan or equal to 200 eV, greater than or equal to 300 eV, greater thanor equal to 400 eV, greater than or equal to 500 eV, greater than orequal to 600 eV, or greater than or equal to 700 eV. As used herein, thetip is contacted with the ion flux when at least part of the tip iscontacted with at least part of the ion flux.

The ion acceleration voltage (also termed the accelerating voltage) issufficiently high that sputtering of the tip occurs. In differentembodiments, the accelerating voltage of the ions is between 550 eV and10 keV, or between 550 eV and 5 keV. Reasonable sputtering rates mayalso be obtained at lower accelerating voltages if the ion currentdensity is sufficiently high (for example in a plasma reactor system).In an embodiment, use of accelerating voltages below those at whichsubstantial implantation occurs can enhance the tip sharpness. Inanother embodiment, the energy of the ion flux is less than about 300keV

The ion beam may comprise positive or negative ions. Ion beam productiontechniques and devices are known to those skilled in the art. Sources ofpositive ion beams include, but are not limited to, ion guns. Suitablepositive ions include, but are not limited to neon, argon, xenon, andhelium ions. The ion current density is sufficiently high to allow thedesired rate of sharpening. In an embodiment, greater ion currentdensity can be attained by alignment of the conductor with the centralportion of the ion beam. In an embodiment, the spot size of the beam isgreater than about 1 micron. As used herein, the conductor tip iscontacted with the ion beam when at least some of the ions in the beamcontact the tip. In one embodiment where the ion flux is provided by anion beam, the acceleration voltage of the beam is between 550 and 5 keVand the voltage applied to the conducting tip is greater than 100 V.

Ion fluxes which are not in the form of ion beams may be provided byplasma reactors or similar pieces of equipment. Plasma reactors canprovide ion current densities on the order of milliamperes per squarecentimeter. A variety of plasma reactor configurations are known tothose skilled in the art. For example, plasma reactors such as parallelplate (diode-type) reactors, triode reactors, and inductively coupledplasma (ICP) reactors are well known for use in plasma etching processesfor semiconductor circuits. These conventional plasma reactors may beadapted to allow separate control of the bias on the conducting tip.

In an embodiment, the conductor is in the form of a wire, nanowire orcolumn having a pointed or narrowed tip and a width or diameter lessthan its length. In another embodiment, the conductor may be in the formof a pyramid or other shape having a pointed tip. For example, methodsfor formation of silicon pyramids are known to those skilled in the art.

Electrically conducting materials suitable for use with the inventioninclude, but are not limited to, metals, carbon, and semiconductors. Inan embodiment, materials suitable for use with the invention have aresistivity less than 10⁷ Ωm. To avoid significant charging effects, theresistance between the apex and ground should be less than several Mohms(megaohms).

In an embodiment, the conductor is selected from group consisting of ametal, a conductive diamond or diamond-like carbon tipped metal, atransition metal carbide tipped metal, a transition metal nitride tippedmetal, a multi-walled carbon nanotube, and a semiconductor

The metal may be a transition metal, a noble metal or ferromagneticmetal. In an embodiment, the metal is a transition metal selected fromthe group consisting of tungsten, molybdenum, chromium, titanium,vanadium, zirconium, niobium, hafnium, and tantalum. In anotherembodiment, the metal is a noble metal selected from the groupconsisting of silver, gold, palladium, platinum, rhodium, iridium,ruthenium, osmium, and rhenium, and combinations thereof. Combinationsof noble metals include alloys such as platinum-iridium alloys. Themetal may also be a ferromagnetic metal selected from the groupconsisting of iron, cobalt, nickel and combinations thereof. The metalmay be polycrystalline. Metal wires useful as conductors for the presentinvention may be of any suitable dimensions known to those skilled inthe art for the intended application. In an embodiment, the diameter ofthe wire is less than 1 mm.

Conductive diamond or diamond-like carbon tipped metals suitable for usewith the invention have carbon coatings sufficiently thick that thesharpening process does not completely remove the coating. It issufficient that the carbon not be completely removed at the apex oralong some distance from the apex. In an embodiment, the thickness ofthe carbon coating is between about 10 nanometers and about 10 microns.Diamond or diamond-like carbon coatings can be deposited by any methodknown to the art, including a variety of chemical vapor deposition (CVD)techniques. Microwave plasma CVD techniques have been used to depositultrananocrystalline diamond films 0.1-2.4 microns thick on sharp singleSi microtip emitters (Krauss et al., 2001, J. Applied Physics, 89(5),2958-2967). Amorphous diamond films have been deposited on Mo tipemitters by pulsed laser deposition (Ding, M. Q. et al, 1997, J. Vac.Sci. Tech. B, 15(4), 840-844).

Metals tipped with transition metal carbides or nitrides are alsosuitable for use with the invention. The transition metal is selectedfrom the group consisting of titanium, zirconium, hafnium, vanadium,niobium, tantalum, chromium, molybdenum, and tungsten. The carbide ornitride coating is sufficiently thick that the sharpening process doesnot completely remove the coating. It is sufficient that the coating notbe completely removed at the apex or along some distance from the apex.In an embodiment, the thickness of the coating is between about 10nanometers and about 10 microns. Transition metal carbide and nitridecoatings can be deposited by any method known to the art, including avariety of chemical vapor deposition (CVD) techniques. When the coatingis formed on a transition metal, the coating may be produced viacarburization or nitridation.

The conductor may also be a multi-walled carbon nanotube. As usedherein, the term “nanotube” refers to a tube-shaped discrete fibriltypically characterized by a substantially constant diameter oftypically about 1 nm to about 100 nm, preferably about 2 nm to about 50nm. In addition, the nanotube typically exhibits a length greater thanabout 10 times the diameter, preferably greater than about 100 times thediameter. The term “multi-wall” as used to describe nanotubes refers tonanotubes having a layered structure, so that the nanotube comprises anouter region of multiple continuous layers of ordered atoms and anoptional distinct inner core region or lumen. The layers are disposedsubstantially concentrically about the longitudinal axis of the fibril.

The conductor may also be a semiconducting material. In an embodiment,the semiconducting material is selected from the group consisting ofsilicon, germanium, or a compound semiconductor comprised of elementsfrom groups III and V of the periodic table. In an embodiment, theconductor is a semiconducting nanowire. As used herein, a nanowire has adiameter or width less than about one micron. In another embodiment, theconductor has a pyramidal shape formed by anisotropic etching.

The field-directed sputtering process of the present invention takesplace under vacuum. As used herein, a vacuum refers to a pressure whichis less than atmospheric pressure rather than to a perfect vacuum.Suitable background pressures, sans sputtering noble gas, are specifiedby ion gun manufacturers and are known by those skilled in the art.Under batch processing in a plasma system the pressures can be on theorder of 1×10⁻⁴ to 1×10⁻² torr during processing. In an ion gun system,the noble gas pressure during sputtering can reach 5×10⁻⁵ torr.Typically, the source of vacuum will be one or more vacuum pumps. Pumpssuitable for obtaining desired vacuum levels are known to those skilledin the art.

In an embodiment, the temperature of the conductor is controlled duringthe sputtering process. In an embodiment, additional heat is applied tothe conductor during the sputtering process to allow at least someannealing of defects generated during sputtering. Suitable temperaturesfor annealing of ion bombardment induced defects are material dependentand known to those skilled in the art. In an embodiment, the conductoris heated to a temperature less than about 1000° C. The conducting tipsproduced by the methods of the invention may also be annealed aftersharpening, but some blunting of the tip may result.

The methods of the invention are capable of producing conductors withultrasharp tips. In different embodiments, the tips produced by themethods of the invention have an apex radius of curvature less than 10nm, less than 5 nm, less than or equal to 2 nm, less than or equal to1.5 nm, or less than or equal to 1 nm. Conductors produced by themethods of the invention are suitable for use as microscope probes (alsoknown as microscope probe tips) and field emitters (also known as fieldemitter tips).

In an embodiment, the invention provides a method for sharpening the tipof an electrical conductor comprising the steps of:

-   -   a. providing a vacuum in a vacuum chamber;    -   b. providing a conductor comprising a tip having an initial        radius of curvature less than 1 micron at its apex, the        conductor being located within the vacuum chamber; and    -   c. simultaneously applying a voltage to the conductor and        contacting the tip of the conductor with an ion beam        characterized by a beam accelerating voltage and comprising        positive or negative ions, wherein the sign of the voltage        applied to the conductor is the same as the sign of the charge        of the ions,        wherein the beam accelerating voltage and the voltage applied to        the conductor are selected so that the tip of the conductor is        sputtered by the ion beam, thereby reducing the radius of        curvature at the apex of the tip to 5 nm or less.

In another embodiment, the invention provides a method for sharpeningthe tip of an electrical conductor comprising the steps of:

-   -   a. providing a vacuum in a vacuum chamber;    -   b. providing a conductor comprising a tip having an initial        radius of curvature less than 1 micron at its apex, the        conductor being located within the vacuum chamber; and    -   c. simultaneously applying a constant positive electric        potential to the conductor relative to the vacuum chamber and        contacting the tip of the conductor with an ion beam comprising        positive ions, thereby reducing the radius of curvature at the        apex of the tip.

The invention provides apparatus capable of sharpening a conductorhaving an initial tip radius of about one micron or less. In anembodiment, the apparatus comprises a vacuum chamber connected to avacuum source, an ion flux source in communication with the vacuumchamber, a sample holder capable of supporting the conductor having atip and capable of being electrically connected to the conductor and asource of electrical potential difference connected between the sampleholder and the vacuum chamber. In one embodiment, the ion flux sourcecomprises a plasma source in combination with a source of ionacceleration voltage. Plasma reactor systems can provide such an ionflux source and also include a vacuum chamber. In another embodiment,the ion flux source is an ion beam. The ion flux source is incommunication with the vacuum chamber so that the ion flux can beintroduced into the chamber. In one embodiment, the ion flux source maybe located within the vacuum chamber. In an embodiment, the sampleholder is adapted to hold the longitudinal axis of the conductor to besharpened at a selected angle (for example a predetermined angle withrespect to the longitudinal axis of an ion beam). The sample holder maybe capable of x, y, z, and rotational adjustments. The apparatus mayfurther comprise a current measuring device connected between the tipand the bias source.

The apparatus may also further comprise a focusing ring. The focusingring functions as an electrostatic lens. In an embodiment, the focusingring is a metal ring positioned between an ion beam source and theconductor to be sharpened so that the ion beam passes through theopening of the ring. The focusing ring is connected to a second sourceof electrical potential difference. During sharpening, a potential isapplied to the ring, resulting in focusing of the beam. The beam isfocused enough to obtain the desired angle of incidence of ions withrespect to the conductor to be sharpened. The voltage applied to thering is less than the ion beam voltage and depends on the size of thering. In an embodiment, the voltage applied to the ring is severalhundred volts. In an embodiment, the amount of focusing ranges from 0 to35 degrees relative to the longitudinal axis of the tip.

The apparatus may also comprise a heater connected to the sample holder.Suitable heating devices for use in controlling the temperature ofsample holders under vacuum are known to those skilled in the art andinclude, but are not limited to resistance-based heaters.

In an embodiment, the invention provides a stand-alone ion sputteringapparatus capable of sharpening a conductor having a sharp tip, theapparatus comprising:

-   -   a. a vacuum chamber connected to a vacuum source;    -   b. an ion flux source in communication with the vacuum chamber;    -   c. a sample holder adapted to hold the conductor to be        sharpened; and    -   d. a source of electrical potential difference connected between        the sample holder and the vacuum chamber.

The invention also provides a scanning probe microscope apparatuscapable of in-situ sharpening of the probe tip. In an embodiment, theinvention provides a scanning probe microscope apparatus comprising avacuum chamber, a scanning probe microscope, an ion beam source and asource of potential difference connected so as to establish anelectrical potential difference between the probe and the vacuumchamber. The scanning probe microscope has a probe comprising a tip andmay be any form of probe microscope known to the art capable ofoperating with a conducting probe. Such scanning probe microscopesinclude, but are not limited to, scanning tunneling microscopes, atomicforce microscopes, magnetic force microscopes, electrostatic forcemicroscopes, scanning voltage microscopes, Kelvin force probemicroscopes, and scanning gate microscopes. The apparatus is configuredso that the ion source and the probe can be positioned to obtain thedesired relationship between the longitudinal axes of the ion beam andthe probe. In an embodiment, the ion source is disposed so that thelongitudinal axis of the ion beam is capable of being substantiallyaligned with the longitudinal axis of the tip. In most cases the tip isremoved from the microscope but not from the vacuum chamber to place itinto alignment with the ion beam. If the probe remains within themicroscope, the microscope may be moved laterally or rotated to alignthe probe with the ion beam. The source of electrical potentialdifference will typically be connected between the probe holder used tohold the probe at the time of sharpening and the vacuum chamber.

In an embodiment, the invention provides a scanning probe microscopeapparatus capable of in-situ sharpening of the probe tip, the apparatuscomprising:

-   -   a. a scanning probe microscope comprising a vacuum chamber and a        probe comprising a tip, the tip apex having an initial radius of        curvature less than 1 micron and the probe being located within        the vacuum chamber;    -   b. an ion beam source in communication with the vacuum chamber;        and    -   c. a source of electrical potential difference connected to        establish a an electrical potential difference between the probe        and the vacuum chamber,        wherein the apparatus is adapted so that probe can be moved        within the vacuum chamber to substantially align the        longitudinal axis of the tip with the longitudinal axis of the        ion beam for sharpening.

In another embodiment, the invention provides an electron microscopeapparatus capable of in-situ sharpening of the field emitter tip. Theapparatus comprises an electron microscope comprising a field emittertip and a vacuum chamber connected to a vacuum source, an ion beamsource and a source of electrical potential difference connected toestablish an electrical potential difference between the tip and thesurrounding vacuum chamber. The source of potential difference can bethe same source of potential difference that causes field emission andtherefore can be part of the microscope. The microscope has a fieldemitter tip and may be any form of electron microscope known to the art,including scanning electron microscopes and transmission electronmicroscopes. The apparatus is configured so that the ion source and thefield emitter tip can be positioned to obtain the desired relationshipbetween the longitudinal axes of the ion beam and the field emitter tip.In an embodiment, the ion source is disposed so that the longitudinalaxis of the ion beam is capable of being substantially aligned with thelongitudinal axis of the tip. In an embodiment, the tip may be rotatedto the side in order to face the ion gun.

In an embodiment, the invention provides an electron microscopeapparatus capable of in-situ sharpening of the field emitter tip, theapparatus comprising:

-   -   a. a electron microscope comprising a field emitter tip and a        vacuum chamber connected to a vacuum source;    -   b. an ion beam source in communication with the vacuum chamber;        and    -   c. a source of electrical potential difference connected to        establish an electrical potential difference between the tip and        the surrounding vacuum chamber,        wherein the apparatus is adapted to allow rotation of the tip so        that the longitudinal axis of the tip is substantially aligned        with the longitudinal axis of the ion beam for sharpening.

When a Markush group or other grouping is used herein, all individualmembers of the group and all combinations and subcombinations possibleof the group are intended to be individually included in the disclosure.Whenever a range is given in the specification, for example, atemperature range, a time range, or a composition range, allintermediate ranges and subranges, as well as all individual valuesincluded in the ranges given are intended to be included in thedisclosure.

As used herein, “comprising” is synonymous with “including,”“containing,” or “characterized by,” and is inclusive or open-ended anddoes not exclude additional, unrecited elements or method steps. As usedherein, “consisting of” excludes any element, step, or ingredient notspecified in the claim element. As used herein, “consisting essentiallyof” does not exclude materials or steps that do not materially affectthe basic and novel characteristics of the claim. Any recitation hereinof the term “comprising”, particularly in a description of components ofa composition or in a description of elements of a device, is understoodto encompass those compositions and methods consisting essentially ofand consisting of the recited components or elements. The inventionillustratively described herein suitably may be practiced in the absenceof any element or elements, limitation or limitations which is notspecifically disclosed herein.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and there is no intention in the useof such terms and expressions of excluding any equivalents of thefeatures shown and described or portions thereof, but it is recognizedthat various modifications are possible within the scope of theinvention claimed. Thus, it should be understood that although thepresent invention has been specifically disclosed by preferredembodiments and optional features, modification and variation of theconcepts herein disclosed may be resorted to by those skilled in theart, and that such modifications and variations are considered to bewithin the scope of this invention as defined by the appended claims.

In general the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences and contexts known to those skilled in the art. The precedingdefinitions are provided to clarify their specific use in the context ofthe invention.

Example 1 Simulation of Beam-Tip Interaction for Perfectly ConductiveMaterials

The electric field surrounding a biased conductor with a sharp tip wassimulated for a two-dimensional case assuming that the material was aperfect conductor, resulting in an absence of electric fields within thetip. The electric potential was also assumed to be constant and presentacross the entirety of the conductor. The simulations demonstrated thatthe electric field was dramatically enhanced near the apex of the tip.The sharper the tip, the more prominent the enhancement.

The ion motion surrounding the biased conductor tip was also simulated.FIGS. 2A, 2B, and 2C present ion path simulation results for a neon ionbeam aligned with the longitudinal axis of the conductor. The ion beamaccelerating voltage was 2000 eV; the neon atoms were singly ionized.FIGS. 2A, 2B, and 2C show simulation results for tip potentials of 100V, 400V, and 800 V respectively. As anticipated, the charged particlesare repelled and higher tip bias results in stronger repulsion.

Sputtering of the conductor tip by the incoming ions was modeled bydividing the simulated tip into finite elements (represented by pixelswithin the image in FIGS. 3-4). The scale of the image is selected basedon physical parameters such that each pixel corresponds to one atomwithin the tip. An extensive series of incoming ions were generatedsequentially with a uniform random distribution in the lateraldirection. The ion paths were simulated until the ion impacted the bodyof the tip or departed from the tip vicinity. Upon impact, the removalof atoms by sputtering was simulated by an algorithm based on theSigmund model (Sigmund, 1973, J. Mat. Sci., 8, 1545). The distributionof energy within the tip and the penetration depth of ions wasindependently computed using the SRIM 2006 software package (Ziegler, etal, 1985, The Stopping and Range of Ions in Solids, Pergamon Press;http://www.srim.org/) and our simulator modeled sputtering from thefundamental theory and these data points. In addition, the tip wasregularly reconstituted and any portions which had become disconnectedfrom the primary body of the tip were fully removed from the system.Also minimization of surface energy was modeled by allowing the motionof atoms within two atomic diameters following each sputtering event ifan energetically favorable position can be located. These simulationsconsider a two-dimensional tip and ignore interaction between ions. Thesimulations were saved in the form of animations. FIG. 3 shows stillimages extracted from an animation for a 2000 eV argon ion (singlyionized) beam and a tip voltage of 400V. In this figure, the images areequally spaced in time. FIG. 4 shows still images extracted from ananimation of a 1600 eV argon ion beam where no tip voltage is applied(control experiment where the tip was electrically grounded). The timespacing is equal between figures. According to these simulations,approximately 20 minutes under an ion current density of 150 μA/cm²should be sufficient for complete sharpening of a tip with an initialradius of curvature of roughly 100 nm.

Example 2 Sharpening of Pt—Ir Tips

The experimental setup consisted of a vacuum chamber with a backgroundpressure of approximately 5×10⁻⁹ torr, which is attached to anultra-high vacuum system containing a scanning tunneling microscope.Within the vacuum chamber was a Model 04-161 ion gun from PhysicalElectronics (advertised current density 300 μA/cm²) controlled by aModel IPS Ion Sputtering Gun Power Supply from OCI VacuumMicroengineering, and a micromanipulator with x, y, z and rotationaladjustments. Tip bias was provided by a Model M107 DC Voltage Sourcefrom Systron-Donner. Transmission electron microscope (TEM) imaging ofthe tips ex-situ was performed with a Philips CM200 microscope.

Platinum-iridium scanning tunneling microscopy probes were purchasedcommercially from Material Analytical Services (MAS). The initial apexradius of curvature of the Pt—Ir tips was roughly 100 nm.

FIGS. 5A through 5F show TEM images of a Pt—Ir tip prior to sputtering(FIG. 5A) and at various stages of the sputtering process (FIGS. 5Bthrough 5F). The tip voltage was 400 V and the neon ion energy was 2000eV. The ion beam was directed substantially parallel to the longitudinalaxis of the tip. FIG. 5A shows the initial status of the tip. A largemass of contaminant is present near the apex, and the tip has a radiusof curvature of roughly 100 nm. After some sputtering (approximately 45min.), as shown in FIG. 5B, the tip appears to have sharpened slightlyand a secondary apex has appeared. Upon further sputtering(approximately 60 min), the side apex appears to have reduced, thoughthe primary apex still has a radius of curvature of approximately 50 nm,as shown in FIG. 5C. It is believed that the tip was not centrallylocated within the beam during these first two sputtering stages. FIG.5D shows that additional sputtering (30 min.) has further sharpened thetip apex and that another secondary apex has appeared. The contaminantseen in this image likely appeared ex-situ while transferring the tipfrom the sputtering system to the TEM. FIG. 5E shows the tip afterfurther sputtering (30 min); the tip has become significantly sharperand the side apex has been reduced significantly. FIG. 5F is a highermagnification image of the tip in FIG. 5E, more clearly illustrating aradius of curvature bordering on the sub-nanometer range. The scalemarkers in these figures are as follows: FIGS. 5A-5B: 100 nm; FIG. 5C:200 nm; FIGS. 5D-5E: 100 nm; FIG. 5F: 20 nm.

The cone angle for the primary apex in FIG. 5B was between 60 and 65degrees. Measurement was made trigonometrically by roughlycircumscribing a right triangle within the tip, measuring opposite andadjacent edges, and computing the arctangent

For control experiments conducted with no tip bias and ion beam energiesbetween 1600 and 2000 eV, the resulting tip radii were between 3 and 10nm, typically between 5 and 10 nm.

Example 3 Sharpening of Tungsten Tips

The sputtering apparatus was as described in Example 2.

Tungsten wires with electrochemically etched tips were prepared byetching polycrystalline tungsten wire in sodium hydroxide solution. Theinitial apex radius of curvature of the tungsten tips was roughly 100nm.

FIG. 6A shows a TEM image of a W tip prior to sputtering; a large flatface is present at the tip apex. FIG. 6B shows a TEM image of a W tipafter sputtering; the tip radius is approximately 1.5 nm, time approx 15min. The tip voltage was 400 V and the neon ion energy was 2000 eV. Theion beam was directed substantially parallel to the longitudinal axis ofthe tip. The scale marker in FIG. 6A is 50 nm; that in FIG. 6B is 20 nm.

For a control experiment with zero tip bias and 2000 eV beamaccelerating voltage the resulting tip radius was approximately 6 nm.

1. A method for sharpening the tip of an electrical conductor comprisingthe steps of: a. providing a vacuum in a vacuum chamber; b. providing aconductor comprising a tip having an initial radius of curvature lessthan 1 micron at its apex, the conductor being located within the vacuumchamber; and c. simultaneously applying a voltage to the conductor anddirecting a flux of ions onto the tip of the conductor, the ion fluxbeing characterized by an acceleration voltage, wherein the sign of thevoltage applied to the conductor is the same as the sign of the chargeof the ions, wherein the acceleration voltage and the voltage applied tothe conductor are selected so that the tip of the conductor is sputteredby at least some of the ions, thereby reducing the radius of curvatureat the apex of the tip to less than 5 nm.
 2. The method of claim 1,wherein a positive voltage is applied to the conductor and the ion fluxcomprises positive ions.
 3. The method of claim 2, wherein the ion fluxcomprises ions selected from the group consisting of neon, argon, xenon,and helium ions.
 4. The method of claim 1, wherein a negative voltage isapplied to the conductor and the ion flux comprises negative ions. 5.The method of claim 1, wherein the absolute value of the differencebetween the acceleration voltage and the potential applied to theconductor is greater than 500 V.
 6. The method of claim 5, wherein theabsolute value of the potential applied to the conductor is greater thanabout 50 V.
 7. The method of claim 6, wherein the absolute value of thepotential applied to the conductor is greater than about 100 V.
 8. Themethod of claim 1 wherein the ion flux is provided by an ion beam. 9.The method of claim 8, wherein the longitudinal axis of the beam issubstantially aligned with the longitudinal axis of the conductor. 10.The method of claim 8, wherein the longitudinal axis of the ion beam isat a non-zero angle to the longitudinal axis of the conductor.
 11. Themethod of claim 1, wherein the ion flux is provided by a plasma reactor.12. The method of claim 1, wherein the conductor comprises a metal. 13.The method of claim 12, wherein the metal is a transition metal selectedfrom the group consisting of tungsten, molybdenum, chromium, titanium,vanadium, zirconium, niobium, hafnium and tantalum.
 14. The method ofclaim 12, wherein the metal is a noble metal selected from the groupconsisting of iridium, platinum, gold, silver, palladium, osmium,rhodium, ruthenium, rhenium, and combinations thereof.
 15. The method ofclaim 12, wherein the metal is a ferromagnetic metal selected from thegroup consisting of iron, cobalt, nickel and combinations thereof. 16.The method of claim 12 wherein the tip of the conductor furthercomprises a conductive diamond or diamond-like carbon coating.
 17. Themethod of claim 12, wherein the tip of the conductor further comprises acoating of a transition metal carbide or nitride, wherein the transitionmetal is selected from the group consisting of titanium, zirconium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, andtungsten.
 18. The method of claim 1, wherein the conductor is amulti-walled carbon nanotube.
 19. The method of claim 1, wherein theconductor is a semiconductor.
 20. The method of claim 1, wherein afterstep c) the radius of curvature of the tip apex is 2 nm or less.
 21. Themethod of claim 1, wherein a constant voltage is applied to theconductor.
 22. The method of claim 21, further comprising repetition ofstep c) with a different constant voltage applied to the conductor. 23.The method of claim 21, further comprising repetition of step c) with atime varying voltage applied to the conductor.
 24. The method of claim1, wherein a varying voltage is applied to the conductor.